Method and system for managing background operations in a multi-layer memory

ABSTRACT

A multi-layer memory and method for performing background maintenance operations are disclosed. The memory includes a plurality of flash memory die having multiple layers, where each layer is made up of flash memory cells having a greater bit per cell storage capacity than then prior layer and each layer may have a plurality of partitions for different data types. A controller managing the flash memory die is configured to identify an idle die and determine if a layer in the die satisfies a background maintenance criterion. Upon identifying a layer satisfying the background maintenance criterion, the valid data from reclaim blocks in the layer is relocated into a relocation block in the same layer until the relocation block is filled and the background maintenance cycle ends.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. App. No. 61/747,916, filedDec. 31, 2012, the entirety of which is hereby incorporated herein byreference.

BACKGROUND

Non-volatile memory systems, such as flash memory, have been widelyadopted for use in consumer products. Flash memory may be found indifferent forms, for example in the form of a portable memory card thatcan be carried between host devices or as a solid state disk (SSD)embedded in a host device. Two general memory cell architectures foundin flash memory include NOR and NAND. In a typical NOR architecture,memory cells are connected between adjacent bit line source and draindiffusions that extend in a column direction with control gatesconnected to word lines extending along rows of cells. A memory cellincludes at least one storage element positioned over at least a portionof the cell channel region between the source and drain. A programmedlevel of charge on the storage elements thus controls an operatingcharacteristic of the cells, which can then be read by applyingappropriate voltages to the addressed memory cells.

A typical NAND architecture utilizes strings of more than twoseries-connected memory cells, such as 16 or 32, connected along withone or more select transistors between individual bit lines and areference potential to form columns of cells. Word lines extend acrosscells within many of these columns. An individual cell within a columnis read and verified during programming by causing the remaining cellsin the string to be turned on so that the current flowing through astring is dependent upon the level of charge stored in the addressedcell.

Flash memory generally provides highest performance when the number ofdata bits per cell is lowest, such as binary flash, also known as singlelevel cell (SLC) flash, that stores 1 bit per cell. Flash memory that isconfigured to store more than one bit per cell, known as multi-levelcell (MLC) flash, can store 2 or more bits of information per cell.While SLC flash memory is generally known for having better read andwrite performance (e.g., speed and endurance) than MLC flash, MLC flashprovides more storage capacity and is generally less expensive toproduce. The endurance and performance of MLC flash tends to decrease asthe number of bits per cell of a given MLC configuration increases.There are continuing challenges in obtaining a desired balance ofperformance, capacity and cost in the design of flash memory devicesusing these types of flash memory cells.

SUMMARY

In order to address the challenges of noted above and provide storageflexibility in a SSD, a multi-layer memory and methods of memorymanagement are provided.

According to a first aspect, a method of performing backgroundmaintenance procedures in flash memory is disclosed. The method includesa controller of a mass storage memory system identifying an idle die ina plurality of flash memory die in the mass storage memory system, whereeach of the plurality of flash memory die include a plurality of layers,and each of the plurality of layers in a particular die having adifferent bit-per-cell capacity. After identifying an idle die, thecontroller determines whether a layer in the plurality of layers in theidle die satisfies a background maintenance criterion. The method thenincludes, in response to determining that a layer satisfies thebackground maintenance criterion, executing a background maintenanceoperation in the layer independently and asynchronously of any otheroperation in other flash memory die managed by the controller. Executingthe background maintenance operation may be accomplished by onlyrelocating valid data between blocks in the layer that meets thebackground maintenance criteria.

In one aspect, a background maintenance criterion may be based on aratio of unused data capacity in free blocks in the layer to a totalunused data capacity of the layer and determining that the layersatisfies the background maintenance criterion may only occur when theratio is less than a threshold ratio. Each maintenance operation mayonly include only relocating enough data from reclaim blocks in a layerto fill one relocation block in the same layer. Also, the identificationof a layer that satisfies the background maintenance criterion mayfurther comprise selecting a partition in a layer that satisfies thebackground maintenance criterion, and then executing the maintenanceoperation by only relocating data between blocks in the partition of thelayer.

In other implementations, the method may include the controller of themass storage memory system identifying multiple of the flash memory dieas idle and independently and concurrently selecting a suitable layer ineach of the identified idle die such that a separate maintenanceoperation may be concurrently executing in multiple idle die backgroundmaintenance cycles independently of one another. A mass storage memorysystem configured to execute the methods described is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system that may implement aspects of theinvention.

FIG. 2 illustrates an example physical memory organization of thestorage device of FIG. 1.

FIG. 3 shows an expanded view of a portion of the physical memory ofFIG. 2.

FIG. 4A illustrates a multi-layer memory organization of the storagedevice of FIG. 1 according to one embodiment.

FIG. 4B illustrates an alternative physical arrangement of themulti-layer memory of FIG. 4A.

FIG. 5 is a flow diagram illustrating a method for operating amulti-layer memory.

FIG. 6 is an example of LBA address space and an associated DLBA addressspace that may be utilized in the system of FIG. 1.

FIG. 7 illustrates an example of LBA to DLBA mapping for data receivedfrom a host.

FIG. 8 illustrates a data relocation operation in DLBA address space andcorresponding updates bocks in physical address space.

FIG. 9 illustrates a second data relocation operation following the datarelocation operation of FIG. 8.

FIG. 10 illustrates an embodiment of the storage device with multi-layermemory organization where each layer includes multiple partitions.

FIG. 11 illustrates a data flow pattern in a layer of a multi-layermemory having multiple partitions.

FIG. 12 illustrates data paths for a three layer memory having twopartitions.

FIG. 13 illustrates data flow paths for a two layer memory having twopartitions.

FIG. 14 illustrates data flow paths for an alternative embodiment of thetwo layer, two partition memory of FIG. 13.

FIG. 15 illustrates a NAND die array with an equal number of die on eachof eight different channels.

FIG. 16 illustrates a synchronous die management arrangement formanaging the NAND die array of FIG. 15 where each of eight controllerinstances manage 4 die on separate channels.

FIG. 17 illustrates an alternative arrangement of the multi-layer memorysystem of FIG. 4A with a memory array and multiple controller instances.

FIG. 17A illustrates an alternative arrangement of the multi-layermemory system of FIG. 4A with a memory array and a single controller.

FIG. 18 illustrates an asynchronous die management scheme implemented bySTAR controllers of the multi-layer memory system of FIG. 17.

FIG. 19 illustrates a NAND die array with an unequal number of die oneach of 4 different channels.

FIG. 20 illustrates an embodiment of LBA grouping to manage an array ofNAND die having an unequal die distribution per channel.

FIG. 21 illustrates a table of one embodiment of allocating LBA groupsto provide an even distribution of excess capacity in the unequal diearrangement of FIG. 19.

FIG. 22 is a block diagram of an LBA splitter for implementing the LBAallocation of FIGS. 20-21.

FIG. 23 is a diagram of possible combinations of concurrent random andsequential program cycles in a multi-layer die.

FIG. 24 is an embodiment of a program cycle chart illustrating thedecision process for moving data between layers or relocating datawithin layers of a multi-layer memory die.

FIG. 25 illustrates an example of interleaving between host write andmaintenance operations in each layer of a multi-layer memory die duringone program cycle.

FIG. 26 is an alternative interleaving pattern of host data writes andmaintenance writes to the example of FIG. 25.

FIG. 27 is an example of overlapping program cycles in a die.

FIG. 28 is an example message flow diagram of a controller in amulti-layer memory.

FIG. 29 is a flow diagram of a block reclaim cycle.

FIG. 30 is a block diagram of an example reclaim cycle according to themethod of FIG. 29.

FIG. 31 is a diagram of example data capacity distribution in a dielayer of a multi-layer memory.

FIG. 32 is a diagram of capacity distribution in the die layer of FIG.31 after sustained background maintenance operations in the die layer.

FIG. 33 is a diagram of a die layer at maximum allowable data capacityaccording to one embodiment.

FIG. 34 is a flow diagram of a background maintenance process in die.

FIG. 35 is a block diagram showing an organization of triple-layer diein a multi-layer memory according one embodiment.

FIG. 36 is a block diagram showing an organization of jointly packageddie pairs in a multi-layer memory.

FIG. 37 is a block diagram showing an organization of separatelypackaged die pairs in a multi-layer memory.

DETAILED DESCRIPTION

A system suitable for use in implementing aspects of the invention isshown in FIG. 1. A host system 100 controls data stored into andretrieved from a physical storage device 102. The storage device 102 maybe a flash device that is embedded in the host, such as a solid statedisk (SSD) drive, an external storage device separate from the host, ora memory card or other removable flash drive that is removably connectedto the host 100, and may communicate through a mechanical and electricalconnector such as connectors 103, 104, or wirelessly, using any of anumber of available wired or wireless interfaces. The host 100 may be adata handling device, such as a tablet computer, mobile phone, personaldigital assistant, home network router, a personal computer (PC) or anyother type of data handling device.

The host system 100 may be viewed as having two major parts, insofar asthe storage device 102 is concerned, made up of a combination ofcircuitry and software. They are an applications portion 105 and adriver portion 106 that interfaces with the storage device 102. In a PC,for example, the applications portion 105 can include a processor 109running word processing, graphics, control or other popular applicationsoftware, as well as the file system 110 for managing data on the host100. In a camera, cellular telephone or other host system that isprimarily dedicated to performing a single set of functions, theapplications portion 105 includes the software that operates the camerato take and store pictures, the cellular telephone to make and receivecalls, and the like.

The storage device 102 contains non-volatile memory 107. Thenon-volatile memory 107 may be configured in a combination of singlelevel cell (SLC) type of flash memory and/or a multi-level cell (MLC)type flash memory. The storage device 102 also includes a host interfaceand controller 108 that may include a processor, instructions foroperating the processor and a logical block to physical blocktranslation tables.

The non-volatile flash memory may be arranged in blocks of memory cells.A block of memory cells is the unit of erase, i.e., the smallest numberof memory cells that are physically erasable together. For increasedparallelism, however, the blocks may be operated in larger metablockunits. One block from each of at least two planes of memory cells may belogically linked together to form a metablock. Referring to FIG. 2, aconceptual illustration of a representative flash memory cell array isshown. Four planes or sub-arrays 200, 202, 204 and 206 memory cells maybe on a single integrated memory cell chip, on two chips (two of theplanes on each chip) or on four separate chips. The specific arrangementis not important to the discussion below and other numbers of planes mayexist in a system. The planes are individually divided into blocks ofmemory cells shown in FIG. 2 by rectangles, such as blocks 208, 210, 212and 214, located in respective planes 200, 202, 204 and 206. There maybe dozens or hundreds of blocks in each plane. Blocks may be logicallylinked together to form a metablock that may be erased as a single unit.For example, blocks 208, 210, 212 and 214 may form a first metablock216. The blocks used to form a metablock need not be restricted to thesame relative locations within their respective planes, as is shown inthe second metablock 218 made up of blocks 220, 222, 224 and 226.

The individual blocks are in turn divided for operational purposes intopages of memory cells, as illustrated in FIG. 3. The memory cells ofeach of blocks 208, 210, 212, and 214, for example, are each dividedinto eight pages P0-P7. Alternately, there may be 16, 32 or more pagesof memory cells within each block. A page is the unit of dataprogramming within a block, containing the minimum amount of data thatare programmed at one time. The minimum unit of data that can be read atone time may be less than a page. A metapage 328 is illustrated in FIG.3 as formed of one physical page for each of the four blocks 208, 210,212 and 214. The metapage 328 includes the page P2 in each of the fourblocks but the pages of a metapage need not necessarily have the samerelative position within each of the blocks. A metapage is the maximumunit of programming. The blocks disclosed in FIGS. 2-3 are referred toherein as physical blocks because they relate to groups of physicalmemory cells as discussed above. As used herein, a logical block is avirtual unit of address space defined to have the same size as aphysical block. Each logical block includes a range of logical blockaddresses (LBAs) that are associated with data received from a host 100.The LBAs are then mapped to one or more physical blocks in the storagedevice 102 where the data is physically stored.

Referring now to FIG. 4A, one embodiment of the storage device 102 ofFIG. 1 is shown having a non-volatile memory 401 that may include threelayers of flash memory cells 402, 404, 406, each layer having adifferent bit per cell capacity. As shown, a first flash memory layer402 may be configured as binary flash having a single bit per cellcapacity. The first flash memory layer is also referred to herein as X1flash. A second flash memory layer 404 may be configured as MLC flash,for example with a two bit per cell capacity, also referred to herein asX2 flash. A third flash memory layer 406 may also be configured as MLCflash, but having a greater bit per cell capacity than the second flashmemory layer 404. In this example the third flash memory layer isillustrated as three bit per cell MLC flash, also referred to herein asX3 flash.

The different flash layers 402, 404, 406 may all be disposed on the samephysical die, each layer may be fabricated on respective separate die,or the layers may be fabricated on a combination of single flash layerdie and combined flash layer die. Although specific bit per cellconfigurations of one, two and three bits are illustrated, othercombinations are contemplated where the first flash memory layer has alower bit per cell configuration than the second flash memory layer andthe second flash memory layer, in turn, has a lower bit per cellconfiguration than the third flash memory layer. FIG. 4B illustrates thenon-volatile memory 401 of FIG. 4A where the first and second flashmemory layers 402, 404 are located on one die 440 and the third flashmemory layer 406 is located on a separate die 442. Also, although threelayers are illustrated in FIGS. 4A and 4B, a greater number of layersmay be implemented in other embodiments.

A dynamic random access memory (DRAM) buffer 408 of the controller 424receives data from the host 100 over an interface 434, such as a SerialAdvanced Technology Attachment (also known as a Serial ATA or SATA)interface. The DRAM buffer 408, under direction from the processor 424of the controller, directs data received from the host at the interface434 to the multi-layer non-volatile memory 401 in the storage device.The DRAM write buffer 408 in the storage device is in communication withthe first and second flash memory layers 402, 404. Also, the first flashmemory layer 402 is in communication with the second flash memory layer404 and the second flash memory layer 404 is in communication with thethird flash memory layer 406.

The controller 424 may be implemented in a single integrated circuitchip and may communicate with the different layers 402, 404, 406 in thenon-volatile memory 401 over one or more command channels 436. Thecontroller may have its own internal bus that links non-volatile memory432 in the controller 424 containing code to initialize (“boot”) thesystem, DRAM 408, interface 434, and circuits 430 that calculate andcheck an error correction code (ECC) for data passing through thecontroller between the multi-layer flash memory 401 and the host.Controller executable code for implementing memory managementinstructions such as described herein may be stored in the multi-layerflash memory 401, for example in the first flash memory layer 402.Additional details on versions of a multi-layer memory device may befound in U.S. application Ser. No. 13/077,487, filed Mar. 31, 2011, andpublished as US Pub. No. 2012/0254574, wherein the entirety of theaforementioned application is incorporated herein by reference.

Referring to FIG. 5, an exemplary method implemented by the controllerof the storage device for moving data into and/or between the flashlayers is disclosed. In one implementation, when data is received from ahost write (at 502), the controller selects a memory layer to receivedata from the host. The selection may be based on whether the data fromthe host that is received at the DRAM from the interface, such as a SATAinterface, is sequential or non-sequential (at 504). For example, if thedata received is sequentially addressed with host LBA addresses, thecontroller may direct the data from the DRAM directly to the secondflash memory layer (at 506). In contrast, if the data is random ornon-sequentially addressed data, that data may be directed from the DRAMstraight to the first flash memory layer (at 508). In oneimplementation, data received at any one time from the host isconsidered to be sequential if the host LBA data addresses of the datareceived are sequential for an amount of data sufficient to fill acomplete metapage of a metablock in the multi-layer memory.

In each of the flash memory layers, the controller of the storage devicemonitors flash memory layer transfer criteria. For example, the flashmemory layer transfer criteria may be whether there are enough freeblocks left in the flash memory layer and a total amount of valid datacontained in the flash memory layer. Once the controller determines thatthe number of free blocks is less than a minimum threshold for the flashmemory layer, the controller next determines whether the amount of validdata in the flash memory layer is above a transfer threshold. When thenumber of free blocks in a flash memory layer is below the minimumthreshold, and the total amount of valid data in that flash memory layerreaches a threshold amount, the controller may cause data from thatflash memory layer to be transferred to the next flash memory layer.Thus, if a criteria for transfer to a next flash memory layer issatisfied in the first flash memory layer, a block of previouslyprogrammed data is selected by the controller from which to copy datainto the second flash memory layer in order to free up the space in thefirst flash memory layer (at 510, 512, 506). Similarly, blocks in thesecond flash memory layer may have data transferred into the third flashmemory layer to free up blocks in the second flash memory layer upon thesecond flash memory layer meeting its criteria for transfer to the nextflash layer (at 516, 518, 522).

The criteria for determining when to transfer data from a source memorylayer to an destination layer, which may include having less than aminimum number of free blocks and a threshold amount of valid data, maybe the same or different for each layer. The last layer, in this examplethe third flash memory layer, would not have a next higher capacity MLClayer to send data to and would therefore not have an assigned transfercriteria.

In one example, the transfer criteria threshold for the first and secondflash memory layers may be identifying that the layer currently containsat least a predetermined percentage of valid data, such as 90%. Inanother embodiment, the transfer criteria may be both that there iscurrently only a threshold number of free blocks in the layer and thatthe layer contain at least a predetermined percentage of valid databefore a block in that layer may be selected for having it datatransferred to the next memory layer. The threshold number of freeblocks may be a minimum number such as one or more free blocks. The dataselected for relocation from the source to the next flash memory layeris preferably from the block having the least recently programmed, or“coldest” host data. The controller may select this block based on orderof programming information maintained for each previously programmedblock in the layer in question.

Illustrated in one implementation, all received host data may beinitially sent to the first flash memory layer before being moved withinthe first flash memory layer, or to a subsequent layer, as set forth ingreater detail below. In another implementation, data received from thehost is directed from the DRAM 408 to the first flash memory layer 402when the received data is in random LBA order (at line 410), or from theDRAM 408 directly to the second flash memory layer, bypassing the firstflash memory layer, when the received data is in sequential LBA order(at line 412). The controller may progressively move data from the firstflash memory layer 402 to the second flash memory layer 404 (at line414) and from the second flash memory layer to the third flash memorylayer (at 416) at appropriate times. Also, garbage collection to createnew free blocks within each flash memory layer is preferably performedsuch that data is recycled within the flash memory layer (at lines 418,420 and 422).

In order to implement the above method and structure described, thecontroller may maintain a linked list of data blocks within each flashmemory layer to record the order in which blocks were programmed in thatlayer. Additionally, the controller may implement an address translationalgorithm referred to as storage address re-mapping (STAR) within eachof the layers to further enhance the efficiency of data transfer andmemory usage.

Although any of a number of known memory management techniques may beused to implement the multi-layer memory system described herein, acontroller configured to utilize STAR techniques is described herein.One advantage of STAR is the ability to increase performance of memorysystems in random write applications, which are characterized by theneed to write short bursts of data to unrelated areas in the logicalblock address (LBA) address space of a device, that may be experiencedin solid state disk (SSD) applications in personal computers. In oneimplementation of the STAR technique, host data is mapped from a firstlogical address assigned by the host to blocks of contiguous logicaladdresses in a second logical address space. As data associated withfully programmed blocks of addresses is made obsolete, a data relocationprocedure is initiated where the controller selects a previously fullyprogrammed block in a layer having the least amount of valid data, orhaving less than a threshold amount of valid data, and relocates thevalid data in those blocks to free up those blocks for use in writingmore data. The relocated data is contiguously written to a relocationblock in the same memory layer in the order it occurred in the sourceblock needing data relocation regardless of the logical address assignedby the host. In this manner, overhead may be reduced by not purposelyconsolidating logical address runs assigned by the host (as in typicalgarbage collection). A storage address table (SAT) is used to track themapping between the logical address assigned by the host and the secondlogical address and subsequent changes in the mapping due to subsequentrelocation in the memory layer.

Concurrently with accepting data from the host, or transferring datafrom a first layer to a next higher bit per cell capacity layer, thecontroller reclaims blocks by copying valid data from previouslyprogrammed blocks having both valid and obsolete data and then recyclingthe blocks from which all the valid data was copied. This blockreclaiming procedure may be in the form of a standard garbage collectiontechnique where groups of data are kept together and consolidated as newdata in the same address run is received, or may be a relocationprocedure, as further discussed below, where data is not consolidatedinto the same address groupings. The garbage collection or relocationprocedure is preferably implemented by the controller independently ineach of the flash memory layers. The valid data copy process, whethergarbage collection or relocation, is implemented within each layer suchthat data moved in the process is preferably maintained in the sameflash memory layer. As shown in FIG. 5, the controller checks to see ifa sufficient number of free blocks exist in the first layer and, if not,performs copy operations on blocks of data within the first layer toconsolidate valid data and create additional free blocks (at 510, 514).This data copy process is independently executed in the second and thirdlayers as well (at 516, 520, 524, 526).

The above-noted method preferentially relocates data within the sameflash memory layer and only moves data to a subsequent layer if thecurrent layer is almost full of valid data. Also, by moving data betweenlayers that comes from the least recently programmed block in a sourcelayer, data tends to be filtered from the first flash memory layer tothe third flash memory layer such that “hot” data tends to reside in thefirst flash memory layer, less actively updated data tends to reside inthe second flash memory layer, and the “cold” data mainly resides in thethird and final flash memory layer. Data is considered “hot” if it isdata that has very recently been updated, as it may be more likely thatthat data is in active use and will be updated again in a short periodof time.

Referring to FIGS. 6-9, an example of the storage address remapping(STAR) address translation algorithm is illustrated. A hypotheticalsection of the host free cluster map in LBA address space 602 and thefree cluster map in the second logical address space, referred to hereinas device logical address space or DLBA address space 604, at a giventime may be represented as shown in FIG. 6. In the LBA address space602, free clusters 606 are dispersed at essentially random locations. Inthe DLBA address space 604, two free blocks 608 are available and thereare three previously programmed blocks 610 having differing numbers ofobsolete (free) clusters 606.

When the host next has data to write to the storage device, it allocatesLBA address space 602 wherever it is available. FIG. 7 shows how thestorage address re-mapping algorithm allocates one of the available freeblocks 170 to be the write block 702, and how each LBA address is mappedto a sequential cluster in the DLBA space available in the write block702. The write block 702 in DLBA space is written to in the order theLBA addresses are written, regardless of the LBA address position. Inthis example it is assumed that the time order in which the host usedfree LBA clusters is the same as the address order for ease ofillustration, however the controller implementing the storage addressre-mapping algorithm would assign DLBA addresses in the write block 702in the time order LBA addresses are used, regardless of the LBA addressnumber order. Data is written in a write block in one or more DLBA runs.A DLBA run is a set of contiguous DLBA addresses that are mapped tocontiguous LBA addresses in the same LBA run. A DLBA run must beterminated at a block boundary in DLBA address space 604. When a writeblock 702 becomes filled, a free block 608 is allocated as the nextwrite block 702.

DLBA blocks are aligned with blocks 704 in physical address space 706 ofthe flash memory, and so the DLBA block size and physical address blocksize are the same. The arrangement of addresses in the DLBA write block702 are also then the same as the arrangement of the correspondingupdate block in physical address space. Due to this correspondence, noseparate data consolidation, commonly referred to as garbage collection,is ever needed in the physical update block.

In common garbage collection operations, a block of logical addresses isgenerally always reassembled to maintain a specific range of LBAaddresses in the logical block, which is also reflected in the physicalblock. More specifically, when a memory system utilizing common garbagecollection operations receives an updated sector of informationcorresponding to a sector in particular physical block, the memorysystem will allocate an update block in physical memory to receive theupdated sector or sectors and then consolidate all of the remainingvalid data from the original physical block into the remainder of theupdate block. In this manner, standard garbage collection willperpetuate blocks of data for a specific LBA address range so that datacorresponding to the specific address range will always be consolidatedinto a common physical block. The relocation operation discussed in moredetail below does not require consolidation of data in the same addressrange. Instead, the relocation operation performs address re-mapping tocreate new blocks of data that may be a collection of data from variousphysical blocks, where a particular LBA address range of the data is notintentionally consolidated.

As mentioned previously, the STAR algorithm operates to ensure that asufficient supply of free blocks is available for the sequential writealgorithm to operate. The STAR algorithm manages the creation of freeblocks by relocating valid data from previously programmed blocks havinga mix of valid and obsolete data to a special write block known as therelocation block 802 (FIG. 8). The previously programmed block currentlyselected for relocation is referred to as the reclaim block.

Referring now to FIGS. 7-8, an illustration of a data relocation processis shown. The storage address re-mapping algorithm designates a freeblock as the relocation block 802, to which data is to be relocated fromselected previously programmed blocks to create additional free blocks.Valid data in the reclaim block (block A of FIG. 7) is selected in theorder that the valid data appears in the reclaim block and relocated tosequential and contiguous addresses in the relocation block 802, toconvert the reclaim block to a free block 608. A corresponding updateblock 704 in the physical address space 706 is also assigned to receivethe relocated data. As with the update block 704 used for new datareceived from the host, the update block 704 for receiving relocateddata will never require a garbage collection operation to consolidatevalid data because the relocation operation has already accomplished theconsolidation in DLBA address space 604.

A next reclaim block (previously programmed block B of FIG. 8) isidentified from the remaining previously programmed blocks asillustrated in FIG. 9. The previously programmed block with the leastvalid data is again designated as the reclaim block and the valid dataof the reclaim block is transferred to sequential locations in the openrelocation block. A parallel assignment of physical addresses in theupdate block 704 is also made. Again, no data consolidation is requiredin the physical update block 704 mapped to the relocation block 802.Relocation operations on previously programmed blocks are performed asbackground operations to create free blocks at a rate sufficient tocompensate for the consumption of free blocks that are designated aswrite blocks. The example of FIGS. 6-9 illustrate how a write block anda relocation block may be separately maintained, along with respectiveseparate update blocks in physical address space, for new data from thehost and for relocated data from previously programmed blocks.Allocation of a new write block for associating new data received from ahost is only performed when a current write block is fully programmed.Similarly, a new relocation block is preferably only allocated after theprior relocation block has been fully programmed. The new relocationblock preferably only contains unwritten capacity, i.e. is onlyassociated with obsolete data ready to erase, or is already erased andcontains no valid data, upon allocation.

In the implementation noted above, new data from a host is associatedwith write blocks that will only receive other new data from the hostand valid data relocated from previously programmed blocks in arelocation operation is moved into relocation blocks that will onlycontain valid data from one or more previously programmed blocks. Inother implementations, the new data and the relocated data may betransferred to a single write block without the need for separate writeand relocation blocks.

The selection by the controller of a previously programmed block as areclaim block may be accomplished by selecting any previously programmedblock on a list of previously programmed blocks that is associated withan amount of valid data that is below a threshold (which may be a fixedthreshold or a variable such as an average amount of valid data for thecurrent previously programmed blocks), or may be accomplished byselecting based on a specific ranking (based on the amount of valid dataassociated with the previously programmed block) of the availablepreviously programmed blocks. Additional details on versions of the STARtechnique usable with the system and methods disclosed herein may befound in U.S. application Ser. No. 12/036,014, filed Feb. 22, 2008 andpublished as US Pub. No. 2008/0307192, wherein the entirety of theaforementioned application is incorporated herein by reference.

The relocation operation described in FIGS. 6-9 relocates relatively“cold” data from a block from which “hot” data has been made obsolete toa relocation block containing similar relatively cold data. This has theeffect of creating separate populations of relatively hot and relativelycold blocks. The block to be reclaimed is always selected as a hot blockcontaining the least amount of valid data. Creation of a hot blockpopulation reduces the memory stress factor, by reducing the amount ofdata that need be relocated.

In an embodiment of the multi-layer memory and method, the controller108 implements the STAR technique illustrated in FIGS. 6-9 in each ofthe respective flash memory layers. In an alternative embodiment of thestorage device of FIG. 4A, a STAR-enabled version of the multi-layermemory system is illustrated, where the first flash memory layer has alesser bit per cell capacity than the second flash memory layer, and thesecond flash memory layer has a lesser bit per cell capacity than thethird flash memory layer. In the example of FIG. 4A, 1-bit, 2-bit and3-bit per cell flash memory layers have been illustrated, although otherincreasing series of bit per cell memory layers may be used, and inother increase increments.

Partitions in Memory Layers

In one embodiment, as illustrated in FIG. 10, each of the layers 1002,1004 and 1006 of the flash memory may include a plurality of partitions,where each partition is assigned to a specific type of data. In theexample of FIG. 10, there are two partitions per layer, a sequentialpartition (1008, 1012, 1016) assigned to sequential data and a randompartition (1010, 1014, 1018) assigned to random data. A NAND block ineach memory layer is permanently allocated to a layer, but may bedynamically allocated to either partition for each cycle of operation.Operation cycles will be described separately in greater detail herein.Although two partitions per layer are shown in FIG. 10, more than twopartitions per layer may be implemented in other embodiments, where eachpartition in a layer is associated with a different data type. Not allpartitions need be present in all layers and the size of a partition ina layer may vary dynamically each operation cycle based on the amount ofdata for the particular data type.

In the following example, sequential data is data having sequential hostlogical block addresses (LBAs) for at least a metapage worth of data ashandled by the flash management algorithm for the storage device. Forexample, if the metapage size managed by the flash management algorithmof the storage device is 128 k, then sequentially addressed runs of atleast 128 k would be considered sequential data and be routed to thesequential partition (1008, 1012, 1016) of a desired layer while datahave less than 128 k of sequential LBAs would be handled by the randompartition of the desired memory layer. The 128 k threshold is providedsimply by way of example as the metapage size may be different fordifferent flash management systems.

In other embodiments, rather than analyzing the LBAs of incoming datafor runs that qualify incoming data as sequential or random, the flashmanagement algorithm in the controller may receive information from thehost on other data type characteristics for the data that would thenpermit other, or additional, partitions to be supported in the differentmemory layers. This information on other data type characteristics maybe for data types such as temporary data, long-term data, read/writelatency, data that is required during a boot process, and so on. Theinformation on these data types may be determined by the controller ormay be provided via “host hints” where the host can pass along aspecific attribute to the storage device with the data using protocolsthat define these other data types.

A flash memory device with layers and partitions creates a storagematrix with two dimensions. The physical memory of the storage devicehas physical memory subdivided into multiple layers 1002, 1004, 1006according to memory type (e.g., single bit per cell (X1), two bits percell (X2) and three bits per cell (X3)), while stored data in each layeris divided into partitions by data type that is determined by thecontroller or provided by the host.

Referring to FIG. 11, a generic structure of one layer 1102 of the 3layer memory with random and sequential partitions of FIG. 10 is shown.The layer 1102 comprises two available write blocks, a sequential writeblock 1104 and a random write block 1106, for data being written fromoutside the layer. Data designated as sequential and random is directedto different write blocks. Instances of both write blocks need not beopen concurrently, or may not be allowed to be open concurrently. Forexample, only one block at a time may be allowed to be open forprogramming in an X3 layer. Similarly, two available relocation blocks,one for sequential data 1108 and another for random data 1110, may existfor data being relocated from within the same layer. Data designated assequential and random when first written to the layer is directed torespective different relocation blocks. Only one relocation block willbe open at a time, and in one implementation, a relocation block 1108,1110 may not be open at the same time as a write block in an X3 layer.

The identities of blocks written as sequential and random blocks areseparately maintained. Two chains 1112, 1114 of blocks for sequentialand random data record the order in which their blocks were programmed.These chains 1112, 1114 are designated as the sequential and randompartitions for the layer as discussed with respect to FIG. 10. A singlepool of free blocks 1116 provides blocks for allocation in thesequential or random partitions as write or relocation blocks.

When the number of blocks in the free block pool reaches a minimumthreshold level, or the volume of valid data in the layer exceeds athreshold amount, such as the LBA capacity of the layer, a maintenanceoperation must be performed in the layer.

A maintenance operation may require data to be relocated within a layerfrom a selected reclaim block 1118, 1120 to the corresponding relocationblock 1108, 1110. In one embodiment, relocation of data in a layer ismanaged such that data remains in the same partition in the layer. Thus,data is normally routed from the random relocation out (RRO) port to therandom relocation in (RRI) port, or from the sequential relocation out(SRO) port to the sequential relocation in (SRI) port, depending on thepartition in which the operation is being performed. However, it ispermitted to configure the data routing to combine relocated data fromboth partitions into a single relocation block, in some embodiments.

Alternatively, a maintenance operation may require data to me moved fromthe least recently programmed block, designated a sequential move block1122 or a random move block 1124, in the selected partition to thecorresponding write block in the following layer (e.g. from X1 to X2).Data is normally routed from the random move out (RMO) port to therandom write in (RWI) port of the next layer, or from the sequentialmove out (SMO) port to the sequential write in (SWI) port of the nextlayer, depending on the partition in which the operation is beingperformed. Although data in one partition is preferably only relocatedto another block in the same layer and partition, or moved to a block inthe same partition in a different layer, it is contemplated that datafrom different partitions may be combined into a single write block inthe next layer, in some embodiments.

Referring to FIG. 12, one embodiment of data flow for the three layermemory having two partitions (referred to herein as a Trio die 1200) ofFIG. 10, is illustrated. The controller may support a data transferalgorithm between layers, herein referred to as a Trio algorithm in athree layer implementation, that would permit use of paths 1202-1232 asshown, but paths 1206, 1214 and 1224 are not used in a Trio die 1200implementation in one embodiment.

Layer 1 (X1) may be used for execution of random write transactions andcontinuation sequential write transactions from the host, althoughstandard sequential write transactions from the host are executeddirectly on layer 2 (X2). Layer 1 therefore maintains both sequentialand random partitions of data blocks. Each maintenance operation to movedata to layer 2 or layer 3 therefore comprises a single type of data,either sequential or random. Random data may be relocated within layer1, but there is no relocation of sequential data in layer 1 in theembodiment of FIG. 12.

Layer 2 maintains both sequential and random partitions, and eachmaintenance operation to move data to layer 3 (X3) or to relocate datawithin layer 2 therefore comprises a single type of data, eithersequential or random. Layer 3 maintains both sequential and randompartitions, and each maintenance operation to relocate data within layer3 therefore comprises a single type of data, either sequential orrandom. Data cannot be moved from layer 3 as there is no next higherlayer. Also, in one embodiment, only a single block in layer 3 may beopen for programming at any time.

The description above with respect to FIG. 12 assumes Trio die 1200having X1, X2 and X3 bit per cell densities as described above. In otherembodiments, a triple layer arrangement of die may be arranged onseparate die, or there may only be a dual layer die. An example of adata flow for a two layer die having only X1 and X2 layers (referred toherein as a Duo2 die 1300) is illustrated in FIG. 13. Similarly, anexample of a data flow for a two layer die having only X1 and X3 layers(referred to herein as a Duo3 die 1400) is illustrated in FIG. 14. Thedata paths in FIGS. 13 and 14 that are the same as those in FIG. 12 aregiven the same numbers for ease of comparison with FIG. 12. In the Duo3die 1400, sequential and random write transactions from the host areboth executed on layer 1, because data cannot be programmed directlyfrom the host to the X3 layer. Layer 1 therefore comprises twopartitions of data blocks. Each maintenance operation to move data tolayer 3 therefore comprises a single type of data, either sequential orrandom. Random data may be relocated within layer 1, but there is norelocation of sequential data in layer 1. Layer 3 of die 1400 isconfigured exactly as for triple layer memory in a Trio die.

For a flash die having a particular layer and partition arrangement in amulti-layer memory there is a fixed number of types of maintenanceoperations that may be triggered by a host write operation. Thecombination of host write and maintenance operations are referred to asprogramming sets. As illustrated in Tables 1-3, one embodiment of allpossible programming sets in the Trio die, Duo2 die and Duo3 diearranged to manage two data partitions are set out. In Tables 1-3, thefirst letter of the programming set denoted random data (R) orsequential data (S) written by the host. The first digit following the Ror S denotes the layer in which the host data is programmed. The 2nd,3rd and 4th digits, when present, denote layers in which data duringsuccessive consequential maintenance operations is programmed. Forexample, the programming set in Table 1 designated “R1233” refers torandom host data directed to the random partition in layer 1 in a Triodie that triggers maintenance operations of moving data from layer 1 tolayer 2, moving data from layer 2 to layer 3 and relocating data withinlayer 3.

TABLE 1 Programming sets in a Trio die Host Host Data MaintenanceOperations Program- Data Write 1st 2nd 3rd ming Set Type Layer OperationOperation Operation R1 Random L1 R11 Random L1 L1 to L1 data relo-cation R12 Random L1 L1 to L2 data move R122 Random L1 L1 to L2 L2 to L2data move data relo- cation R123 Random L1 L1 to L2 L2 to L3 data movedata move R1233 Random L1 L1 to L2 L2 to L3 L3 to L3 data move data movedata relo- cation S1 Sequential L1 S13 Sequential L1 L1 to L3 data moveS133 Sequential L1 L1 to L3 L3 to L3 data move data relo- cation S2Sequential L2 S22 Sequential L2 L2 to L2 data relo- cation S23Sequential L2 L2 to L3 data move S233 Sequential L2 L2 to L3 L3 to L3data move data relo- cation

TABLE 2 Programming Sets in a Duo2 die Host Host Data MaintenanceOperations Program- Data Write 1st 2nd 3rd ming Set Type Layer OperationOperation Operation R1 Random L1 R11 Random L1 L1 to L1 data relo-cation R12 Random L1 L1 to L2 data move R122 Random L1 L1 to L2 L2 to L2data move data relo- cation S2 Sequential L2 S22 Sequential L2 L2 to L2data relo- cation

TABLE 3 Programming Sets in a Duo3 die Host Host Data MaintenanceOperations Program- Data Write 1st 2nd 3rd ming Set Type Layer OperationOperation Operation R1 Random L1 R11 Random L1 L1 to L1 data relo-cation R13 Random L1 L1 to L3 data move R133 Random L1 L1 to L3 L3 to L3data move data relo- cation S1 Sequential L1 S13 Sequential L1 L1 to L3data move S133 Sequential L1 L1 to L3 L3 to L3 data move data relo-cation

With respect to the features of multiple layers in each die, where eachlayer has a plurality of partitions associated with data of a certaintype or certain attribute, in different embodiments, the partition towhich a block belongs may be determined by the controller or by the hostproviding an attribute associated with LBAs of data stored in the block.Alternatively, the memory system may determine what partition a blockbelongs to based on an attribute of the state of the memory systemduring a defined operation. The multi-layer, multi-partition structureand controller may handle routing of blocks of data from a host torespective desired initial layers and partitions, as well as relocationof data within layers and relocation of data between layers. A pluralityof blocks in a layer may be written with data from a source outside thelayer and each of the plurality of blocks may be written in a differentpartition in the layer.

Selection of blocks for relocating data or moving data duringmaintenance operations in a multi-layer, multi-partition die may beaccomplished using the same criteria as for multi-layer die describedwith respect to FIGS. 4-5 above. Also, relocation and movement of datamay be from different partitions than the partition in which the hostdata that triggered the one or more maintenance operations is directed.Because the partitions are dynamic and free blocks in a layer areassigned on demand to whatever partition needs an additional free block,the maintenance operation may look only at a die layer as a whole forblocks to move to another layer or relocate within the layer.

In addition to the block selection criteria noted previously,alternative block selection criteria for maintenance operations arecontemplated. For selecting a block to move between layers, instead ofselecting the least recently programmed block based on the order inwhich the block was programmed, the move block may be selected based onthe age of the data in the previously programmed block. In other words,selecting the previously programmed block having the oldest average databased on age data for each data element (sector, cluster or othersub-block data granularity that is being tracked) in each previouslyprogrammed block. The age of data in the previously programmed blocksmay be maintained by the controller in a control data structure as partof a storage address table(SAT) or other lists that map logical and/orphysical addresses. In one implementation, the age information for thedata may be a relative age, where the age for a particular piece of datain a block is recorded as a number representing the order it was firstwritten into the storage device from the host. The controller would,assuming that transfer criteria such as those discussed previously hadbeen met, then select the block having the oldest average age of data.An advantage of this alternative technique for selecting the move blockis that it may do a more thorough job of segregating “cold” and “hot”data between the flash memory layers. Using the least recentlyprogrammed block as the sole criteria might miss older data that hasbeen relocated within the flash memory layer and thus is now part of amore recent previously programmed block

In another alternative embodiment, referring again to FIG. 11, insteadof having separate reclaim blocks 1118, 1120 and move blocks 1122, 1124in a memory layer, the separate reclaim and move blocks may be replacedwith only a move block (not shown) in each layer, where a portion of thedata in the move block for the layer is dedicated for transfer to thenext higher capacity layer and a remaining portion of data is relocatedto the respective relocation block 1108, 1110 for the layer. In thisalternative embodiment, selection of the combination move/reclaim block(in each of the first and second layers for a Trio die) may beaccomplished either by selecting the least recently programmed block, orby selecting the block with the oldest average data. The amount of datato transfer from the selected combination move/reclaim block may be afixed amount or a percentage, for example fifty percent of the validdata in the block may be designated for transfer to the next layer andthe remaining fifty percent may be relocated within the layer. Theselection of which fifty percent to transfer from the identifiedmove/reclaim block may be made based on the age of the data as trackedby the controller in the control data structure for the memory system.

In other alternative embodiments, additional considerations regardingwhether or not to transfer data to a next layer may be overlayed withthe programming order or data age considerations noted above. Forexample, in one embodiment it may also be beneficial to increaseperformance by maintaining older (i.e. “colder”) data in a lower bit percell capacity flash memory layer if that data is frequently read. Readcounts for data or blocks of data may also be maintained in themulti-layer, multi-partition memory such as in the control datastructure for the memory system. The read count information may be usedto supplement the data transfer criteria such that data from the leastfrequently programmed block, or the block with the oldest average age ofdata, will not be selected for transfer to a next higher bit per celllayer if the number of read operations on that data is above apredetermined threshold.

Asynchronous Die Management

The multi-layer memory with partitions in each layer has been describedabove in the context of a single memory with a single controller,however the concept of multi-layer and multi-partition per layer memoryis scalable to an array of memory die such as illustrated in FIGS. 15and 16 where an array 1500 of the memory die are shown. For example, inthe 4×8 array 1500, 4 NAND die are arranged on each of 8 differentchannels 1502.

In one implementation, there may be 8 separate controllers in thestorage device, each controller assigned 4 die in a row such that thedie assigned to the controller are each on a different channel. Thecontrollers may be separate physical controllers each with their ownprocessor, or one or more processors each executing different softwareinstances of the controller software for respective die associated withthe controller instance. FIG. 16 illustrates synchronous die managementarrangement where each controller (NAND manager) 1602 controls 4 die inparallel on respective different channels. An arrangement such as shownin FIG. 16 can provide for very efficient processing of sustainedsequential writes because a metablock sized to stripe data across eachof the die managed by the controller can efficiently programsequentially addressed data in parallel.

In a synchronous die management algorithm, multiple NAND die managed bythe same controller perform program and erase operations synchronously.In such an architecture, a single controller handles multiple die, eachof the die assigned to a separate channel, where multiple blocks frommultiple die are linked into a metablock, which becomes the basic unitof NAND program management. In this example, data is programmed inparallel in units of a metapage of size 128 KB, where 32 KB is writtenacross each of the four die on their separate controller channels.Transactions for writing data at all levels in the system, fromfront-end to NAND, have the same data length as this NAND metapage, i.e.128 KB.

In the synchronous architecture, once a write operation begins, all fourdie are considered “busy” until the slowest one of the die in that 4 dieset has finished writing its 32 KB of data. This may block or preventthe host from initiating other operations (programming or read activity)on the remaining die in the set while the slowest die. Another issuethat may arise from tying 4 separate die together on separate channelsunder one controller for a 128 KB write is that a complete writerequires a metablock made up of a block from each of the 4 die. So, ifone die has fewer good blocks than another die in the 4 die group, thecontroller only has access for write operations up to the capacity ofthe die in the group with the least number of good blocks. Any extragood blocks in other of the die for that group, more than the number ofgood blocks in the die with the least good blocks for that group, cannotbe used for a write operation because of the 128 KB write size.

In instances where the received write commands are for smaller, randomdata writes, the efficiency is reduced in a synchronous die managementarchitecture. For example, if there is a sustained random write, thelarge transaction size may cause blocking of the host interface due tosignificant differential between the times the individual NAND managers(controller instances) 1602 remain busy with their respectiveoperations. Furthermore, the large metablock size and parallel channeldie control of FIG. 16 may also be inefficient for mixed reads andwrites with small amounts of data. Each write transaction in FIG. 16will keep 4 die busy and thus blocks 4 die from any pending readtransactions.

In one embodiment, instead of the synchronous controller arrangementdiscussed above, where multiple blocks from multiple die are linked intoa metablock that a controller programs or erases synchronously, anasynchronous architecture is contemplated. As shown in FIGS. 17, 17A and18, in the asynchronous architecture each controller is assigned die ona single channel (FIG. 17) or a single controller may be assigned to alldie on all channels (FIG. 17A). The memory system 1700 in FIG. 17includes many of the same features of that of FIG. 4A, but illustratesan array 1702 of NAND die where columns of NAND die associated with asingle controller channel 1704 are controlled by a separate controller1706. Each controller instance may handle data programming on adie-by-die level and each instance of the STAR controller can select thebest die on the single channel for a particular operation based on itsavailability.

In the multiple controller embodiment of FIG. 17, the controllers 1706may use the STAR address translation algorithm 1708 discussed above withrespect to FIGS. 6-9 and utilize a program scheduling algorithm 1710discussed in greater detail below that incorporates the asynchronous diemanagement algorithm 1712 discussed herein and the Trio algorithm 1714for managing movement of data to, within and between die memory layersthat has been set forth above. In the memory system 1700A with thesingle controller in the embodiment of FIG. 17A, where a singlecontroller 1706 manages all NAND die on one or many channels 1704, thecontroller is the same type of controller with the same algorithms(1708, 1710, 1712, 1714) as the multi-controller embodiment of FIG. 17.The single controller embodiment simply manages the full address mappingfor die and channel for all data in the NAND array 1702 rather thanreceiving data in a specific LBA address range associated with aparticular controller/channel as discussed with respect to themulti-controller embodiments having a separate controller per channel.For ease of illustration, and to avoid repetition, the featuresdescribed below are generally explained in the context of the multiplecontroller arrangement of FIG. 17. It should be understood, however,that all of the features disclosed herein are equally applicable to thesingle controller embodiment of FIG. 17A.

In one embodiment of the present invention, the STAR controller 1706includes an asynchronous die algorithm 1712, in which data programmingparallelism and write transactions throughout the system are managed inunits of the maximum programming parallelism achievable within one die,which is typically 32 KB in a 2-plane die. Each STAR controller 1706 isassociated with die on only one channel 1704, rather than die onseparate channels as in the synchronous architecture. In this manner,multiple die can operate fully in parallel when required, or can operateasynchronously with each performing unrelated operations. Additionally,the asynchronous operation permits the STAR controller for each set ofdie it manages on the same channel to select the die that is best suitedfor receiving the data, for example the die having the shortest queue ofdata to be written. In the embodiment of FIG. 17A, the single controller1706 manages all die an all channels and can select any die that is bestsuited for receiving data.

The asynchronous die algorithm may provide reduced latency for executionof host commands and improved performance with workloads havingirregular I/O size or mixed read/write characteristics. While there maybe timing, accessibility and blocking (i.e. reduced operation blocking)benefits of the asynchronous die algorithm over synchronous diealgorithms, independent management of I/O operations in NAND memoryincreases average die utilization and average performance for bothrandom and sequential workloads. The asynchronous die algorithm mayenable better management of the multiple partitions in each layer of adie because of the greater ease in managing partitions in one die at atime over coordinating partitions in multiple die simultaneously (e.g.four die in the synchronous algorithm discussed above). Further, theremay be improved availability of die in the memory system to readoperations when a combination of read and write operations are beingreceived

Asynchronous operation of each die associated with a STAR controllerallows all good blocks within each die to be active. The performance ofeach die is therefore maximized by making use of the full physicaloverhead available to it, thereby maximizing the performance of thedrive. An example of the potential for increased utilization of blockswithin a given die may be seen when comparing a synchronous diealgorithm, such as illustrated in FIG. 18 where each STAR controller isassigned to 4 different die on the same channel (See also FIG. 17). Forease of illustration, only two of the plurality of STAR controllers 1706of FIG. 17 are reproduced in FIG. 18 and several elements omitted.

As data comes in to the memory system, the memory system directs thedata to the buffer for the STAR controller associated with theappropriate host LBA range and the STAR controller will select the bestdie in the group of die it is managing. In FIG. 18, four separate writecommands 1802, each with 32 KB of data in this embodiment in the LBArange associated with the particular STAR controller, are directed tothe STAR controllers STAR0 1706A and STAR7 1706B. Utilizing theasynchronous die management algorithm 1712, the STAR0 controller 1706Aselects any available individual NAND die from the die 1804 it ismanaging on the same channel for each command and routes data for eachwrite command 1802 to a respective die 1804. The selection of the NANDdie 1804 by the STAR0 controller 1706A is independent of the dieselection of the separate die managed by the STAR7 controller 1706B orany other controller in the memory system for data directed to thoseother controllers. As shown in FIG. 18, the order of the die selected bythe STAR0 controller 1706A is NAND die 2, 4, 1 and 3 for the datacommands 1-4, respectively. The STAR controller 1706B, based on thestatus information it has for the NAND die it manages, selected adifferent order of NAND die 1804 (NAND die 4, 1, 3, 2).

The Trio algorithm 1714 in each STAR controller provides statusinformation for each die to the asynchronous die algorithm 1712,indicating whether host data program transactions may be issued for thedie. Status information may be based on the number of queuedtransactions for the die within the NAND driver 1806, and whether therequired interleaving of maintenance copy operations with host dataprogram operations has been met. Based on this status information, eachinstance of a STAR controller may utilize its asynchronous die algorithm1712 to issue a host data write transaction from its front-end queue toany die managed by that STAR controller that is ready to receive it.

The die selection criteria implemented by the asynchronous die algorithm1712 may include the STAR controller selecting a die only if the die'sstatus information provided from the Trio algorithm 1714 indicates thatthe required interleaving of maintenance copy operations with host dataprogram operations of the same class (e.g. data type of random orsequential) as the pending transaction has been met. Other criteria,which may be used alone or in combination, may include selecting a dieonly if the number of outstanding transactions for the target class(e.g. the queue for random write transactions) does not exceed aspecified limit; selecting the available die with the lowest number ofoutstanding transactions for the target class; preferentially selectingthe available die with the lowest program cycle type active for thetarget class; and/or selecting the available die with the lowest logicalfullness.

While a specific STAR controller manages data with LBAs only within aspecific subset of host LBA address space, there is no correlationbetween LBA and NAND die number within a STAR controller instance.Similarly, there is no correlation between the NAND die numbers used bysuccessive STAR controllers to sequentially program successive LBAmetapages within a stripe of LBAs spanning the separate die managed bytwo STAR controllers. Although each STAR controller independentlymanages its own die on one particular controller channel in oneembodiment of an asynchronous architecture, the asynchronous diealgorithm can also achieve full parallelism across STAR controllers fordata transfer and NAND command execution. A NAND read or programoperation on an LBA metapage can be executed in each of the STARcontrollers, with fully synchronous data transfer and NAND accessoperations. This allows fully synchronous execution of read and programoperations relating to an LBA address stripe spanning all STARcontrollers, such as occur in sequential read and write modes ofoperation.

Read and program operations relating to two successive LBA stripesspanning an LBA range associated with a STAR controller implementingasynchronous die management can be executed concurrently in two die onthe same channel, but not fully in parallel. Data transfers to or fromthe two die must be serialized, because they are sharing the samechannel. The read or program operations in the two die are thereforeoverlapped, but not fully parallel.

The asynchronous die operation described above is an enabling featurefor operation with an unequal number of die per STAR controller. Thetotal physical over provisioning in the design of a multi-layer memorysystem such as described below can therefore be defined with agranularity of one die, or die-pair, which can minimize the cost of adrive for a specified level of performance.

The type of inputs received by the asynchronous die algorithm 1712running in each of the STAR controllers may include read, write and trimcommands. In embodiments where the asynchronous die algorithm is used ina multi-layer drive having multiple partitions per layer, the writecommands received may be further divided into write commands fordifferent data types, such as the random and sequential data typesdescribed above. In one embodiment, the execution of certaintransactions may be ordered such that read transactions are immediatelyprocessed, while write and trim commands are processed in the orderreceived. In embodiments where a die metapage is 32 KB, a readtransaction may be for any multiple of 2 KB up to a maximum of one diemetapage, write transactions are one metapage and a trim transaction hasa data payload length of one metapage.

Although the asynchronous die algorithm may be implemented in a memorysystem having multi-layer and multi-partition memory system, in otherembodiments, the asynchronous die management algorithm may also beimplemented on simpler memory systems having single layer die with orwithout partitions.

Unequal Numbers of Die Per Channel

One benefit of a flash memory system that utilizes an asynchronous diemanagement protocol, such as described above, is the ability to expandthe memory one die at a time such that a memory system may have unequalnumber of die per channel. In other words, one instance of a STARcontroller may be associated with a different number of die than anyother instance of a STAR controller in the memory system such that thenumber of die per STAR may be unequal. As noted previously, in someflash memory systems comprising multiple memory die, high writeperformance is achieved by linking blocks from multiple die intometablocks to which data is programmed with increased parallelism, suchas with synchronous die management architecture. The total number of diein the system is therefore a multiple of the number of die across whichmetablocks are formed, and it is not possible to precisely control thephysical capacity over provisioning, also referred to as physicaloverhead.

Referring again to FIG. 15, an example of an equal number of die perchannel is seen in the NAND array 1500, where each of 8 channels 1502includes 4 NAND die. In a synchronous die management scheme, for examplethe synchronous die management algorithm employed in the controllers1602 such as shown in FIG. 16, an array of 8 controllers 1602 eachmanages 4 die from the array 1500, one die each on different channels1502 so that the write operation by each controller 1602 utilizes ametapage that stripes across all 4 NAND die for increased parallelism.Because this type of synchronous die management requires multiple die,it is not able to handle unequal numbers of die per channel.

Using the asynchronous die management algorithm allows the number of diein a flash memory system to be controlled with a precision of one, byallowing operation with unequal numbers of die attached to the memorychannels. The die level granularity of control via the asynchronous diemanagement algorithm may help with managing the total physical overheadof a SSD. The total physical overhead of a flash storage device isdefined as (Physical_capacity−LBA_capacity)/LBA_capacity.

Some physical overhead is required in a flash storage device for anumber of reasons. It allows space for accumulation of obsolete data inthe drive to reduce the amount of data relocation during garbagecollection. Physical overhead also allows space for storage of controldata and system data, as well as provides spare capacity for errormanagement. Additionally, physical overhead can allow use of additionaldata storage layers using NAND technology with fewer bits per cell thanin the primary data storage layer, for example the X1 layer in a Duodrive, such as an SSD consisting of two die layers such as the Duo2 orDuo3 die noted above, or the X1 and X2 layers in a Trio drive having 3die layers.

The random write capability for a memory system is generally related thephysical overhead available in the memory system. Thus, additionalphysical overhead can help improve the performance of a memory system.In a maintenance operation such as garbage collection, the controllerneeds to relocate valid data in a block and the overhead time to do thisoperation depends on the percentage of blocks that contain valid data.With more over provisioning (e.g. a higher ratio of physical space tological space), less valid data may need to be relocated from a givenblock.

Other tasks in a memory system that can benefit from having additionalphysical capacity (beyond a fixed logical capacity) include tasks suchas parity schemes where the error tolerance of the memory system can beimproved. If, for each page written an exclusive or (XOR) operation isperformed and the result stored, then the system can be tolerant of lossof a complete page (where the error correction code (ECC) storing thedata is only capable of correcting for a smaller loss of data). Excessphysical capacity may be used for this or other tasks. If additionalphysical storage is needed for a system for reasons such as parity, theability to include unequal numbers of die per channel allows an increasein physical capacity tailored to the specific need and without needingto, as in the synchronous algorithm discussed above, add a full set of 4die that are necessary for the metablock writes in that example. Theability to add only a single die to any one channel may provide cost andspace savings.

An example of an unequal die per channel arrangement is shown in FIG.19. In the example of FIG. 19, the memory array 1900 is a four channelarray (see channels C0-C3) with 4 NAND die 1902 in channels C1-03 and 5NAND die 1902 in channel C0. In this example, it is assumed that aseparate STAR controller is assigned to each channel and that the STARcontrollers incorporate the asynchronous die algorithm capabilities asshown and described with respect to FIGS. 17-18. For ease ofillustration, each die in FIG. 19 is labeled with the STAR controllernumber (S0-S3) that manages the particular die 2002.

Utilizing the asynchronous die management algorithm, multiple die mayoperate asynchronously on one channel managed by one STAR controller.Because different STAR controllers in a particular memory system neednot have same logical capacity, die count can be increasedasymmetrically in STAR controllers and physical overhead can be tuned inmultiples of one die as shown in FIG. 20. In order to handle theallocation of logical block addresses (LBA) among the STAR controllersand adjust the physical overhead for the whole memory system whenphysical capacity is being increased in one, or fewer than all, STARcontrollers, a scheme is for LBA allocation to the STAR controllers iscontemplated.

Referring to FIGS. 20 and 21 one such LBA allocation scheme for use inthe NAND array 1900 with unequal die allocation shown in FIG. 19 isillustrated. In order to manage the LBA allocation, LBA metapages 2002are striped across successive STAR controllers in sets called LBA groups2004. Even though the extra physical capacity in FIG. 19 is from a die1904 only added to the first channel C0, the entire memory array 1900benefits from the over provisioning by reallocating the fixed amount oflogical address space equally over each of the die.

The structure of an LBA group in FIG. 20 mirrors the asymmetry in dieallocation to STAR controllers (i.e. to each channel) in FIG. 19. Asillustrated, a discontinuity in LBA allocation to successive STARcontrollers occurs at the boundary between LBA groups 2004. In thismanner, the fixed number of LBA addresses is assigned at the front endto each of the STAR controllers such that the physical overheadpercentage is the same for each STAR controller. The allocation of LBAaddresses 2002 for the example of FIG. 19 is also illustrated in FIGS.21-22 in the form of host LBA (in die metapages) number 2102, theassociated STAR controller ID 2104 (in this example S0-S3) and theoffset 2106 of the LBA metapage 2104 in the respective STAR controller.The management of the LBA allocation to the various STAR controllers inthis unequal die per channel arrangement may be implemented by an LBAsplitter function 2200 executed by a processor in the memory device thatoutputs the STAR ID and the STAR offset for data associated with areceived LBA (for each die metapage).

Program Cycle Scheme

A program cycle is a burst of predetermined program operations withinone die in which a single full programming set, as described above inTables 1-3, is performed. It causes a defined amount of host data to beprogrammed in write blocks in one die-layer and predeterminedconsequential maintenance programming operations to be performed ifnecessary in each of the other die layers.

Maintenance operations required in each of the layers of a multi-layermemory need to be known in advance to allow optimum scheduling of hostdata write and maintenance operations. Existing schemes for maintenanceoperation control are reactive to host behavior, not proactive.

In one embodiment, the program cycle method described herein is a schemefor scheduling program operations in each of the memory layers to reducethe maximum latency experienced when programming data from a host.Program scheduling is managed in programming bursts called programcycles, which may span all memory layers. Maintenance operations arepredetermined to create free blocks to replace those consumed in writingnew data to a layer during a program cycle and each program cycleinterleaves the writing of new host data with the predeterminedmaintenance operations. During a program cycle, a fixed amount of hostdata may be programmed, interspersed equally among maintenanceoperations. This is done to prevent a host command from experiencing anunacceptably long response time from the memory system under any rarecombination of circumstances. In one implementation, during a programcycle, bursts of host data and data for maintenance operations areinterleaved at a steady rate, according to the current operating stateof the drive.

A die, such as the Trio die described above with two partitions perlayer, normally has two program cycles concurrently active, of which onemust be for each different partition. Here one cycle may be active forrandom host data and the other for sequential host data. As a die isgenerally only able to execute a single operation at a time due tophysical limitations, the interleaving of the two program cycles in adie is also explicitly controlled.

In one embodiment, to implement the program cycle method, where hostdata operations and maintenance operations are arranged in predeterminedcombinations to account for various possible scenarios, a commonprogramming unit is used. A program unit of a single metablock for theparticular die layer would be acceptable if a programming operation onlyaffected one layer of a die, or if the amount of data in a metablock wasconstant between all layers of a die. However, because a programmingoperation may lead to cascading operations on different layers, wherethere are differing bit per cell densities in the different layers, ametablock of data in one layer is less than a metablock of data in ahigher bit-per-cell density layer in the same die. Thus the unit of dataa program cycle may handle can either be a metablock of host data if thehost data write operation will not result in maintenance operationsoutside the layer in which the host data is received, or can be aprogram unit sized to work efficiently with all of the layers of thedie.

As used herein a program unit (PU) is constant amount of data for eachtype of die, i.e. Trio, Duo2 and Duo3. A PU comprises the fixed numberof metapages which is the lowest common multiple of the number ofmetapages in the individual metablock types in the die layers (X1, X2 &X3). This allows data that is written to a layer or is moved betweenlayers during a program cycle to always be a multiple of the metablocksize within the layer in which it is programmed. An example of the PUsize for specific die types is illustrated in Tables 4-6 for Trio, Duo2,and Duo3 die.

TABLE 4 Definition of PU in a Trio die Nand Program Unit (PU) Die TypeWordlines/Block Metablocks Metapages X1 layer eX3 86 6 516 X2 layer eX386 3 516 X3 layer eX3 86 2 516

TABLE 5 Definition of PU in a Duo2 die Nand Program Unit (PU) Die TypeWordlines/Block Metablocks Metapages X1 layer eX3 86 6 516 X2 layer eX386 3 516

TABLE 6 Definition of PU in a Duo3 die Nand Program Unit (PU) Die TypeWordlines/Block Metablocks Metapages X1 layer eX3 86 3 258 X3 layer eX386 1 258The lowest common number of metapages that consist of a whole number ofmetablocks in each of the layers of a Trio die and a Duo2 die is 516metapages, while the lowest common number in the Duo3 die is 258metapages. Thus the PU for the Trio and Duo2 is 516 metapages and 258metapages for Duo3.

Utilizing the PU appropriate to the particular die, the STAR controllerfor the particular die can determine which of a predetermined number ofprogram cycle types should be executed to handle the next host write ofone PU's worth of data to the memory device. Referring to FIG. 23, thefull set of possible program cycle types within an embodiment of theTrio algorithm for 3-layer and 2-layer memory die is illustrated. Theseprogram cycle types correspond to the programming sets defined in Tables1-3 above. Because a die can execute a random program cycle and asequential program cycle concurrently in one embodiment, cycles ofopposite class have been arbitrarily paired in each die in FIG. 23. Theparticular cycles paired up at any given time in a die of an operatingmemory will depend on the type of host data writes that are pending andthe state of the various layers of the die (e.g. the number of freeblocks and ratio of valid data to LBA capacity and so on).

Referring now to FIG. 24, a chart 2402 of the decision process for aSTAR controller implementing the Trio algorithm, based on the die layerstatus (for example the number of free blocks and amount of valid datain relation to LBA capacity in each layer), is illustrated. The programcycle types listed in FIG. 24 correspond to the programming sets definedin Table 1. The STAR controller preferably selects the program cycletype that incorporates the fewest number of die layers possible, withinthe constraints of the die layer status as shown in FIG. 24. As notedpreviously, the decision criteria for whether to move or relocate datain any given layer may differ or be the same. Also, the threshold valueof the particular criteria (e.g. the minimum number of free blocks) maybe the same or differ in each layer. The term “N” in N blocks or N PU'sof FIG. 24 refers to the number of relocation blocks that must becompletely filled in a reclaim cycle to achieve a net gain of one freeblock. The data paths which can be used to link die-layers in a selectedprogram cycle are described in FIG. 12. As noted previously, the STARcontroller may have two program cycles active in a die, one for eachpartition, at a time, with each of the two program cycles beinginterleaved with the other.

Program Scheduling Scheme

As noted above, the program cycle needed for the next host data write isdetermined prior to the next host data write. When a program cycle hasbeen defined according to the available program cycle types in FIG. 24,the precise numbers of metapages to be programmed in each of thedie-layers in the course of the program cycle are known in advance.However, in one embodiment, a die normally has only two concurrentlyactive program cycles (one for each partition), which have beeninitiated completely asynchronously. The interleave ratio betweenmaintenance programming operations and host data programming operationsfor a program cycle which is being initiated therefore has to takeaccount of outstanding program operations in the program cycle which isalready active. No change is made to the interleave ratio for the activeprogram cycle.

A program interleave scheme according to the following principles isadopted for interleaving operations for two program cycles in a die tocreate a flow of host data and maintenance program operations. A programcycle in a die may be initiated only after a previous cycle of the sameclass (random or sequential) has completed. A program cycle may beinitiated in a die when a program cycle of the opposite class (random orsequential) is already active in the die, and the two cycles mayoverlap.

When overlapping program cycles have maintenance operations in the samedie-layer, the maintenance operations in the first cycle must completebefore maintenance operations in the second cycle can be started. Whenoverlapping program cycles have maintenance operations in differentdie-layers, the maintenance operations in the die layers may beperformed concurrently. Host data write operations in a program cyclebeing initiated must be interleaved with both maintenance operations inthe same cycle and outstanding maintenance operations in the activecycle that is being overlapped.

When a program cycle is initiated, separate interleave ratios aredefined for the minimum number of maintenance operations in eachdie-layer that must be performed per host data write operation in thecycle. During execution of a program cycle, a host data write operationis permitted only if the number of maintenance operations alreadyperformed in each die-layer per previous host write operation is greaterthan or equal to the required interleave ratio for each die layer.

Also, if a program cycle is initiated while a background cycle is activein the other partition, the interleave ratio established for the newprogram cycle must include all maintenance operations in that cycletogether with all outstanding maintenance operations in the backgroundcycle, as discussed in greater detail below in the section regardingbackground cycles.

An object of the program scheduling algorithm executed by a STARcontroller for each die it is managing is to schedule together thewriting of host data and maintenance operations during each programcycle in a manner that provides as uniform a rate of writing host dataas possible. This is accomplished by essentially reducing the rate thathost data is written so that host writes and maintenance writes areevenly distributed. The program scheduling algorithm in the controllerwill permit the controller to balance the multiple PU's that are beingwritten (host or maintenance writes) in a die during a program cycle oroverlapping program cycles. The program scheduling algorithm cancompensate for unknown rates at which host data may be received at theparticular die and for concurrently running program cycles on theparticular die. As part of the program scheduling method, the controllerwill permit maintenance operations to proceed in a program cycle if hostdata is delayed so that the maintenance operations may sometime getahead of the required maintenance operation to host write ratio for agiven program cycle, but will limit host write operations to be withinthe required ratio of maintenance operations to host write operations sothat there should not be a need to catch up on maintenance operations ina die.

An example of interleaving between host write and maintenance operationsin each layer during one program cycle (in this example a R1233 cycle asset out in FIGS. 23 and 24) is shown in FIGS. 25 and 26. In FIG. 25, itis assumed that each of the X1, X2 and X3 layers are on separate die andthe PU for the transaction is 516 metapages. Thus, the programming inall of the layers in FIG. 25 can happen simultaneously. The amount ofhost data written into the X1 layer, however, is interleaved with thesimultaneous maintenance activities in the X2 and X3 layers of thedifferent die so as to spread out the X1 host data write over the timethat the slower maintenance process in the X3 layer requires. Thus, theone PU of host data is not written all at once, but is instead spacedout in time in FIG. 25 and interleaved with the X2 and X3 maintenanceoperations required for the program cycle so that the interleave ratiofor each layer (X1, X2, X3) of page writes of host data to page writesof maintenance data during the specific program cycle is maintained andnot exceeded. The program scheduling algorithm in the controller willallow the controller to reduce the rate of host page writes (here to X1)within the PU such that the interleave ratios for each layer neverexceed the number host write operations to maintenance operations withina program cycle. If all three layers are on the same die, as illustratedin FIG. 26, then the writes to each layer are interleaved, but in astaggered manner because of the general physical limitation that onlyone operation can execute at a time on a single die.

An example of the overlapping of program cycles in a particular die isshown in FIG. 27. In FIG. 27, it is assumed that the die is a Trio die(3 layers) and the decision process used by the controller managing theparticular die for selecting which program cycle to implement is that ofFIG. 24. The serial programming cycles shown in FIG. 27 are a R1233programming cycle PC1, an S23 programming cycle PC2 and an R123programming cycle PC3. For purposes of this example, PC1 is initiatedwhen no other program cycle is active in the die, and therefore operateswith interleave ratios relating only to itself. In this case 1 PU ofdata is written to the X2 layer and 3 PU's of data are written to the X3layer. Thus the X2:host interleave ratio is 1 and the X3:host interleaveratio is 3.

Program cycle PC2 of type S23 is initiated (arbitrarily) after 200metapages of host data have been programmed in PC1, and overlaps PC1.The number of maintenance operations in X3 that must be interleaved withhost data program operations in PC2 is 1464 metapages, comprising 948metapages outstanding in PC1 and 516 metapages in PC2. Thus the X3:hostinterleave ratio is 2.84 (=remaining maintenance metapages into X3/hostdata into X2=1464/516=2.84)

Program cycle PC3 of type R123 is initiated when PC1 completes, but itsmaintenance operations in X3 cannot be started until those for PC2 havecompleted. However, maintenance operations in X2 can start immediately,as no overlapping program cycle is performing maintenance in X2. At thispoint, the X2:host interleave ratio is 1 and the X3:host interleaveratio is 2.

If the arrival rate of host data matches or exceeds the rate ofperforming maintenance operations required by the interleave ratios fora program cycle, the cycle proceeds with uniform interleaving of hostdata metapages and bursts of maintenance metapages. In this case, theprogram cycle is complete when the last metapage of maintenance data hasbeen programmed.

However, if the cumulative arrival rate of host data is lower than canbe accommodated by the program cycle in accordance with its requiredinterleave ratios, all required host data will not have been programmedwhen the last metapage of maintenance data has been programmed. Theprogram cycle is therefore not complete. The program cycle remains openwith its host data write block open, but with maintenance inactive.Further host data can be written to the program cycle without need forinterleaved maintenance operations. The program cycle is only completewhen all required host data has been programmed, that is, a fullmetablock or full PU has been programmed, depending on program cycletype.

An advantage of the program cycle scheme discussed above is that itpermits advanced determination of the necessary maintenance operationsto free up a programming unit's worth of space in advance of asubsequent host write. As part of the program schedule scheme, a set oflinked layers within the plurality of die in which one program cyclewill be performed are identified. The linked layers may be in one dieor, in a plurality of die. The process may allow a layer within a die tobe incorporated in only one program cycle for a specific type of data(e.g. random or sequential) and may allow a layer within a die to beincorporated in two program cycles for different types of data.

As noted above, it is advantageous to know in advance what maintenanceoperations are required in each of the layers of a multi-layer memory toallow optimum scheduling of host data write and maintenance operations.Existing schemes for maintenance control are generally reactive to hostbehavior, not proactive. In one embodiment, host data programmingoperations and maintenance programming operations to relocate data to adifferent block are determined in advance for a sequence of operationsabout to be started. Each programming sequence incorporates a fixedlength of host data (e.g., one program unit) and variable lengths ofmaintenance data (multiple program units). Required interleave ratiosfor all types of maintenance programming relative to host dataprogramming are determined in advance and host data programming ispermitted only when sufficient maintenance programming has beencompleted to satisfy all interleave ratio requirements. This mechanismallows host data program operations to be uniformly scheduled amongmaintenance program operations. Thus, a minimum level of responsivenessto host data is maintained on a program cycle by program cycle basis.

A method of interleaving the programming of host data into a memorysystem and the relocation of data between blocks in the memory systemmay include predetermining the amount of host data and the amount ofrelocated data to be programmed during an operating sequence. For eachtype of block to be programmed with relocated data, predetermining itsrequired program interleave ratio relative to host data. During aprogram cycle, the controller may determine the current cumulativeprogram interleave ratio relative to host data for each layer beingprogrammed allowing host data to be programmed if the current cumulativeprogram interleave ratio for each layer exceeds its required programinterleave ratio.

This cumulative program interleave ratio is the ratio of page writes forthe host data to maintenance operation writes during a particularprogram cycle. The controller will permit the number of maintenanceoperation writes to exceed the interleave ratio if there is a delay inreceiving host data for a program cycle, but will not allow host datawrites to exceed the interleave ratio. When there are concurrentoverlapping program cycles happening in a layer, the particular requiredinterleave ratio may be a modified interleave ratio, such as thatcalculated in FIG. 27 for the program cycle PC2 (ratio=2.84) whichoverlapped an ongoing program cycle in the second partition for the dielayer. In those instances, the program scheduling algorithm executed bythe controller will compare the cumulative interleave ratio during acycle to the modified interleave ratio to make sure write operations ina cycle do not exceed that modified interleave ratio.

Referring to FIG. 28, a portion of the memory system of FIG. 17 is shownillustrating how the asynchronous die algorithm 2802 and Trio algorithm2804 operate together within each STAR controller 2806 to scheduleprogram and copy transactions which are to be executed by the NANDDriver 2808. The Trio algorithm 2804 defines and initiates programcycles and manages maintenance operations within them. Programoperations in NAND die are triggered by both host data programtransactions (write transactions 2810) issued by the asynchronous diealgorithm 2802 and copy transactions 2812 issued by the Trio algorithmfor maintenance operations. These are converted by the NAND Driver 2808into descriptors to control the datapath and NAND components.

The Trio algorithm predetermines any maintenance operations which haveto be performed together with the host data program operations, and theprogram scheduling algorithm manages the interleaving of host dataprogram 2816 and maintenance copy transactions 2812 for execution by theNAND Driver 2808.

An enable signal 2814 from the Trio algorithm 2804 signals when theasynchronous die algorithm 2802 can interleave host data programtransactions with the maintenance copy transactions 2812 being createdby the Trio algorithm 2804. Host data transactions 2816 are enabled whenthe ratio of the number of maintenance metapage copy transactions issuedby the Trio algorithm 2804 divided by the number of host data programtransactions 2816 within the current program cycle are greater than orequal to the required interleave ratio for each layer.

Program transactions may have higher priority than copy transactionswithin the NAND Driver 2808, and therefore take precedence for executionover queued copy transactions. This means that host data programtransactions 2816 issued by the asynchronous die algorithm 2802 whenenabled by the Trio algorithm 2804 will be executed as the nexttransaction, ahead of pipelined maintenance copy transactions 2812already present in the NAND Driver 2808. The Trio algorithm alsoprovides die status 2818 and maintenance status 2020 information toallow the asynchronous die algorithm 2802 to enable backgroundoperations in a Trio die.

With respect to scheduling operations within a STAR controller 2806, thefollowing transactions are passed to the NAND Driver by the STARcontroller 2806: read, program, erase and copy. The execution prioritiesfor these transactions in the NAND driver may be as follows: anoutstanding read transaction for a die has absolute priority over anyother transaction type for that die. Outstanding program, erase & copytransactions for a die are executed with the following preferredpriority: program, erase, copy.

The NAND driver 2808 may over-ride this preferred priority order whenconstructing a set of descriptors for parallel operation in one track.Transactions of the same type for a die-layer must complete in the orderthey are issued to the NAND driver. Transactions of different types fora die-layer may complete in a different order from that in which theyare issued to the NAND driver. Execution of program steps fortransactions of the same type for different die-layers should cycleequally between the die-layers.

In one embodiment, while NAND operations initiated by a sequencerdescriptor cannot be interrupted, transactions sent to the NAND drivermay be interrupted in some cases. Transactions in X1 or X2 die-layerscannot be interrupted, however a copy transaction for an X3 die-layerthat is in progress may be interrupted by any outstanding read orprogram command for the same die. The outstanding read or programcommand may be executed at the end of the program step currently inprogress.

With respect to responses from NAND driver 2808, the following responsesmay be provided by the NAND Driver to transactions sent to it by a STARcontroller 2806. For a read transaction, a single response is sent whenall data is in DRAM. For an erase transaction, a single response is sentwhen the block erase is complete. For a program transaction in X1 or X2,the following responses may be sent: a first response is sent when alldescriptors relating to execution of the transaction have been loaded tothe datapath hardware; and a final response is sent when the programoperation is complete. For a copy transaction in X1 or X2, the followingresponses may be sent: a first response is sent when the data has beenread and transferred to DRAM and all descriptors relating to executionof the transaction have been loaded to the datapath hardware; and afinal response is sent when the program operation is complete. For acopy transaction in X3, the following responses may be sent: a firstresponse is sent when the data has been read and transferred to DRAM andall descriptors relating to execution of the first program step for thetransaction have been loaded to the datapath hardware; a second responseis sent when the first program operation is complete; a third responseis sent when the second program operation is complete; and a finalresponse is sent when the third program operation is complete. Indifferent embodiments, a maximum number of transactions of each type perdie that may be outstanding may be set before a final response to thetransaction has been issued by the NAND driver.

As noted above, an instance of the STAR controller has no knowledge ofthe existence or state of any other instances of STAR controllers.However, the NAND Driver schedules issuance of descriptors to thesequencer hardware with the objective of maximizing data throughput inNAND memory and minimizing idle time in individual NAND die. Operationsgrouped in a track should have similar expected duration. In oneimplementation, the following rules should be observed. Cached programand read commands should be used wherever possible, to allow concurrentdata transfer and NAND program or read operations in a die. Descriptorsin a track should relate to operations of a single type, i.e. read,program or erase. Descriptors in a track relating to program operationsshould be for a single NAND type, i.e., X1, X2 or X3. Descriptors in atrack relating to program operations should be for the same page type(lower or upper) in X2, or same program step type (LM, foggy or fine) inX3.

Reclaim Cycle Scheme

As noted above, a program cycle causes a defined amount of host data tobe programmed in write blocks in one die-layer and predeterminedconsequential maintenance programming operations to be performed ifnecessary in various die layers for each die. Two types of maintenanceoperations, moving valid data between die layers and reclaiming blocksby relocating valid data within a layer from select closed blocks(previously programmed blocks with obsolete data) designated reclaimblocks have been described in particular.

Referring again to FIGS. 8-9, the general movement of data in a reclaimoperation in a STAR controller is shown, where valid data from variousselected reclaim blocks is moved contiguously into a relocation block802. In embodiments where each die layer in a multi-layer die includesmultiple partitions, the reclaim operation relocates data from blocksassociated with data from a particular partition into one or morerelocation blocks assigned exclusively to data from that partition inthe same layer. The reclaim blocks that are emptied of valid data maythen be erased and available in the free block pool for the die layerand later used for either partition depending on current demand forspace for a particular data type.

In some types of NAND memory, there may be a requirement that no blocksin a layer of the memory are left partially programmed at the end of anoperation to reclaim capacity occupied by obsolete data in blocks inthat layer. Also, it is desirable to obtain a net gain of at least onefree block for every group of reclaim operations for use in achievingthe efficiencies of the program cycles described above.

A reclaim cycle scheme is defined herein as a variable duration reclaimoperation having a duration sufficient to satisfy both the criteria ofleaving no relocation block partially programmed at the end of a reclaimoperation and obtaining a net gain of at least one free block. In oneembodiment, a reclaim cycle is a set of maintenance operations within aprogram cycle, which causes data to be relocated from one or morereclaim blocks within a die-layer to one or more relocation blockswithin the same die-layer. The number of reclaim blocks erased must beat least one greater than the number of relocation blocks allocated andfilled, that is, a reclaim cycle must show a net gain of at least onefree block.

Referring to FIG. 29, a reclaim cycle may include the controller for theparticular die allocating a block from the free block pool for theappropriate die layer as a relocation block (at 2902). All valid datafrom a string of selected reclaim blocks is relocated to the currentrelocation block (at 2904). When the relocation block becomes filled (at2906), if the number of blocks in the free block pool is one or morehigher than the number at the start of the reclaim cycle, the reclaimcycle is complete (at 2908). If either the current relocation block hasnot been filled, or there has not been a net gain of one free block,then the relocation of data from reclaim blocks to relocation blockscontinues.

Thus, for relocation within a layer, the controller preferably performsdata relocation operations from a plurality of source blocks in a memorylayer to fill an integral number of destination blocks in the samememory layer, which creates at least one free block in the memory layer.This is in contrast to a maintenance operation for moving data betweenlayers of a die. In that instance, a data move operation from aplurality of blocks in a source memory layer to fill one block in adestination memory layer, which creates at least one free block in thesource memory layer

In the reclaim cycle, an integral number of relocation blocks must befilled, but data may be relocated from a non-integral number of reclaimblocks. An example of a reclaim cycle performed according to the methoddescribed in FIG. 29 is illustrated in FIG. 30. Referring to theprogramming timeline, a first relocation block (Relocation Block 1) isallocated from the free block pool and valid data from Reclaim Block 1is relocated into Relocation Block 1. When Reclaim Block 1 is then freedup after all its valid data has been moved, the net gain of free blocksis 0 because one free block was used for Relocation Block 1 and freeingup Reclaim Block 1 merely maintains the status quo. Accordingly, thereclaim cycle in FIG. 30 continues with allocation of Relocation Block 2and Relocation Block 3, and the contiguous remapping of valid data fromother designated reclaim blocks fills Relocation Block 3 while freeingup four Reclaim Blocks (Reclaim Blocks 1-4) for a net gain of 1 freeblock. It should be noted that to completely fill Relocation Block 3,some, but not all, valid data from Reclaim Block 5 was necessary. Oncethe criteria of completely filling an integer number of relocationblocks and achieving a net gain of one free block has been accomplishedpart way through relocation of the valid data from Reclaim Block 5, thereclaim cycle stops. The remaining valid data in Reclaim Block 5 may berelocated in a subsequent reclaim cycle. While the example of FIG. 30shows a situation where 5 reclaim blocks are needed to fill up 3complete relocation blocks and achieve a net gain of one free block, thereclaim cycle scheme will adapt to different ratios of reclaim andrelocation blocks to achieve the two criteria. Differing amounts ofvalid data that need to be moved from different reclaim blocks willchange the number of reclaim and relocation blocks necessary to completea reclaim cycle.

Reclaim cycles are used within both program cycles and backgroundcycles. In a background cycle, as described below, only one relocationblock is filled and there is no requirement for a net gain of one freeblock.

Background Cycle Scheme

In addition to the maintenance operations (e.g. block reclaim in a givendie layer or movement of data from one layer to a next layer) that takeplace during a program cycle while write commands are pending, amodified background cycle scheme is contemplated in one embodiment. Thepurpose of background operations in a die is to increase the numbers offree blocks in its die-layers to be greater than the minimum numbersrequired to support foreground operations in the die. This allowssubsequent foreground operations to be executed with fewer maintenanceoperations, with consequently higher performance for the memory system.

Background operations can only be executed in an idle die. A die is idleif no host data write transactions are queued for execution in the dieand no maintenance operations are active in the die. Background cyclesmay be concurrently active in any number of idle die, however only onebackground cycle at a time may be active in any given die. In a memorysystem architecture arranged with an asynchronous die managementalgorithm, for example one of the STAR controllers illustrated in FIG.17 and discussed in detail above, the STAR controller can initiate andmanage background cycles in multiple idle die concurrently.

A background cycle operates within a single die-layer. It creates freespace in the die-layer by performing reclaim operations as describedabove. One background cycle causes one relocation block (e.g. ametablock) within the selected die layer to be filled with valid datarelocated from two or more reclaim blocks. Note that a single backgroundcycle in a layer of a die may not result in the net gain of a freeblock, but that sustained execution of background cycles in a die-layerproduces progressive increase in the number of free blocks. A backgroundcycle performs relocation of data within a single die-layer. Data canonly be moved between die-layers within program cycles in oneembodiment, which are foreground operations.

A background cycle in a die may be initiated when the die idle conditionis signaled from the asynchronous die algorithm and no background cycleis currently active in the die. Background cycles preferably will not beinitiated in a die layer where the total amount of valid data for thedie layer exceeds a maximum amount, or where the fraction of unused datacapacity in the die layer that exists as free blocks (a “reclaimedratio”) is above a predetermined threshold.

The reclaimed ratio may be calculated according to the formula:reclaimed ratio=F/(F+O) where F is the amount of free space in freeblocks for a layer and O is the amount of obsolete data in all closed(fully programmed) blocks in the layer. As shown in FIG. 31, the totaldata capacity in a die-layer 3102 is the capacity available for datablocks that have been closed after being fully programmed. It is thetotal physical capacity of the die-layer minus capacity occupied byreserved blocks 3104. These reserved blocks comprise blocks forinformation other than host data (e.g. control data), data blocks thathave been partially programmed and are still open, and the minimumnumber of free blocks necessary for foreground operation.

This total data capacity may comprise capacity occupied by valid data inclosed data blocks (V), obsolete data in closed data blocks (O), andcapacity in excess free blocks (F). Excess free blocks are free blocksin excess of the minimum number of free blocks that are required forforeground operation. Background operations convert obsolete datacapacity in closed data blocks into capacity in excess free blocks byperforming reclaim operations.

As noted above, the ratio F/(F+O) in a die-layer is known as thereclaimed ratio for the die-layer, and is the fraction of the unuseddata capacity that has been converted to free blocks. A maximum valueexists for the reclaimed ratio, which is applied whatever the volume ofvalid data in the die-layer. A background cycle cannot be initiated ifthe maximum reclaimed ratio has been reached, and background operationsin the die-layer must stop until further obsolete data is created byoverwrite or trim operations, or free blocks are consumed by furthervalid data writes. The reclaimed ratio may be a static number designedinto the memory device. Use of a reclaimed ratio as a criterion ininitiating a background cycle makes the background operation moreefficient by avoiding operations in layers where there is hardly anyobsolete data, preventing lengthy operations with minimal gain. Thereclaimed ratio threshold may be the same or different for each layertype in a die.

FIG. 32 shows the capacity distribution in the same die-layer as FIG. 31after a subsequent period of sustained background reclaim operations.Here, some obsolete data capacity (O) has been converted into free blockcapacity (F) as compared to the distribution in FIG. 31. In oneembodiment, background operations also cannot be initiated in adie-layer if the amount of valid data in the die-layer exceeds a maximumlimit 3302, such as shown in FIG. 33. Thus, FIG. 33 illustrates the samedie-layer as in FIG. 32 at a point where the amount of valid data hasreached a predetermined maximum allowable limit 3302. The physicalcapacity of a memory device must exceed the logical capacity.

FIG. 34 illustrates one method of implementing a background cycle in amulti-layer die. The process of FIG. 34 may be performed for each diemanaged by a STAR controller such that multiple background processes maybe concurrently active for a STAR controller. If the controllerdetermines, for example from asynchronous die algorithm, that a die isidle (at 3402), then the controller will evaluate die-layers within theidle die to see if its reclaimed ratio is less than a predeterminedthreshold for its layer type. The controller selects a die layer, in oneimplementation, on a round robin basis such that if, in a Trio die, theX1 layer of a particular die was selected for a background cycle in thelast round, then the controller will first evaluate the X2 die layer thenext time the die is idle and the controller is select a die layer toevaluate (at 3404). The controller will calculate the reclaimed ratio asnoted above for the selected layer and compare the result to thepredetermined maximum threshold for the die layer (at 3406, 3408). Ifthe reclaimed ratio exceeds the threshold, the next die layer isselected (at 3410) if there are any layers in the die that have not beenchecked in this cycle. If the reclaimed ratio is less than thethreshold, then the controller will perform one background reclaim cyclein the selected layer (at 3412). Thus, a die-layer being evaluatedshould only be selected if its reclaimed ratio is less than a definedmaximum reclaimed ratio for its layer type and background cycles areterminated if no die layer can be selected.

Also, the selection of a partition within a die layer may be made usingthe same principles discussed above with respect to selection of apartition for movement and relocation in foreground maintenanceoperations in program cycles. For example, if there are no closed blocksin the partition, or if the partition contains less obsolete datacapacity in its closed blocks, then that partition is not selected inthat die layer. Generally, for a selected die layer the controller mayselect the partition with the lowest average valid data count per closedblock. Within the selected partition of the selected die layer, thereclaim block selected (i.e. the closed block with obsolete data) may bethe one with the least amount of valid data.

If a host data program transaction is issued to a die while a backgroundcycle is active on the die, host data program operations have higherpriority in the NAND Driver than the data copy operations used in thebackground cycle. Host data programming therefore has priority, andfurther relocation of background cycle data takes place only when nohost data program transactions are pending.

If a full PU of host data has been written and a program cycle completedbefore a background cycle in the same die has completed, a new programcycle is initiated in the die. If the program cycle does not incorporatemaintenance operations in the die-layer in which the background cycle isactive, the outstanding operations in the background cycle areinterleaved by the NAND Driver alternately with maintenance operationsin the same die. Background operations are therefore automaticallyinterleaved with host data program operations without being explicitlyincluded in the interleave ratio of the program cycle. However, if theprogram cycle does incorporate maintenance operations in the die-layerin which the background cycle is active, the outstanding operations inthe background cycle should be completed before maintenance operationsin the layer are started, and the background operations may be includedin the calculation of interleave ratio for the die-layer in the programcycle.

Die Package Configurations

In a three layer solid state drive (SSD), also referred to as a Triodrive, there are multiple options for the physical organization of thethree layers of storage into separate units managed by the Trioalgorithm, and the organization of units into separate packages. Asdiscussed above, a Trio die is a single eX3 die, which is managed as anindependent region of physical storage by an instance of a STARcontroller, and in which three fixed sets of blocks are configured foruse as X1, X2 and X3 blocks. FIG. 35 shows flash memory configurationfor a Trio drive with 8 STAR controller instances, with three layers ofstorage blocks in each Trio die 3502 and multiple Trio die in each of 8packages 3504.

All Trio die 3502 in one package 3504 are connected to the samecontroller channel, and are managed by the same STAR controller, such asillustrated in FIG. 17, in one embodiment. If a second bank of 8packages 3504 is required in a higher capacity drive, they may besimilarly connected to channels 0 to 7. The second bank of packages canbe a capacity extension for STARs 0 to 7, or can be a set of independentSTAR controller instances 8 to 15 (not shown).

As illustrated in FIG. 36, the three layer structure may also beachieved using jointly packaged Trio die-pairs. A Trio die-pair 3602 isone eX2 die 3604 and one eX3 die 3606 forming a unit, which is managedas an independent region of physical storage by the STAR controller forthe respective channel, and in which three fixed sets of blocks areconfigured for use as X1, X2 and X3 blocks. A drive with 8 STARcontroller instances, with three layers of storage blocks in each Triodie-pair and multiple Trio die-pairs in each of 8 packages 3608 isillustrated in FIG. 36. Both die 3604, 3606 forming a Trio die-pair 3602are within the same package 3608. All Trio die-pairs 3602 in one package3608 are connected to the same controller channel, and are managed bythe same STAR controller instance. If a second bank of 8 packages isrequired in a higher capacity drive, they should be similarly connectedto channels 0 to 7. The second bank of packages can be a capacityextension for STAR controllers 0 to 7, or can be a set of independentSTAR controllers 8 to 15.

As illustrated in FIG. 37, the three layer structure may also beachieved using separately packaged die-pairs. A Trio die-pair 3702 isone eX2 die 3704 and one eX3 die 3706 forming a unit, which is managedas an independent region of physical storage by the STAR controller, andin which three fixed sets of blocks are configured for use as X1, X2 andX3 blocks. The system in FIG. 37 shows a drive with 4 STARS, rather thanthe 8 assumed for the arrangements of FIGS. 35 and 36 above, with threelayers of storage blocks in each Trio die-pair 3702 and each Triodie-pair spread over 2 packages 3708. The two die forming a Triodie-pair are in different packages 3708, and each package comprises onlyeX2 die or only eX3 die. All die in one package are connected to thesame controller channel, and are managed by the same STAR controller.Each adjacent pair of packages is managed by the same STAR. If a secondbank of 4 packages is required in a higher capacity drive, they shouldbe similarly connected to channels 0 to 7. The second bank of packagescan be a capacity extension for STAR controllers 0 to 3, or can be a setof independent STAR controllers 4 to 7.

A system and method have been disclosed for a multi-layer,multi-partition memory system. In one embodiment, a modified version ofa STAR controller (which may be separate hardware controllers ormultiple instances of software running on each of one or more hardwarecontrollers) is designed to operate with three layers of storage in eachdie or die-pair having X1, X2 and X3 NAND flash blocks. A subset of thedisclosed algorithms may also be used in a two layer drive. The overallthree layer STAR algorithm described above, for example with respect toFIG. 17, is used to manage a dedicated set of flash die in a solid statedisk drive, such as die on a particular controller channel. Multipleinstances of the three layer STAR algorithm will exist within an SSDsystem, each managing a separate set (channel) of flash memory die.

As disclosed above, the operations of the three layer STAR algorithm maybe made up of four algorithms: 1) an asynchronous die algorithm; 2) aTrio algorithm; 3) a program scheduling algorithm; and 4) an addresstranslation algorithm:

Asynchronous Die Algorithm

Data programming parallelism and write transactions throughout thesystem are managed in units of the maximum programming parallelismachievable within one die, which is typically 32 KB in a 2-plane die.Multiple die can operate fully in parallel when required, or can operateasynchronously with each performing unrelated operations.

The asynchronous die algorithm is designed to provide reduced latencyfor execution of host commands and improved performance with workloadshaving irregular I/O size or mixed read/write characteristics.

Based on status information received from the Trio algorithm, theasynchronous die algorithm may issue a host data write transaction fromits front-end queue to any die that is ready to receive it. Availabilityis indicated by the depth of queues of host data write transactionsissued to the NAND Driver. Transactions should be issued preferentiallyto those die with the shortest queues.

Trio Algorithm

The memory space in a Trio die is divided into three memory layersaccording to NAND block type (X1, X2 and X3), and also into twopartitions according to data type (random and sequential).

Within the layer and partition structure of a Trio die, data from thehost is written to layers 1 and 2. Maintenance programming operationsmay also be required in one or two of the memory layers in the die as aconsequence of data being written from the host.

A sequence of predetermined program operations known as a program cyclecauses a defined amount of host data to be programmed in write blocks inone layer of a die and predetermined consequential maintenanceprogramming operations to be performed if necessary in one or two layersof the same die. During a program cycle, programming of host data anddata for maintenance operations is interleaved at a steady rate, tocreate best uniformity of host response times and ensure that no hostcommand can experience an unacceptably long response time under any rarecombination of circumstances.

Maintenance operations create free blocks in a layer for reuse inwriting new data to the layer, and may comprise moving a fixed amount ofdata from one layer to another or relocating data from one block toanother within a layer. Maintenance is performed as foregroundoperations or background operations when the die is idle.

Program Scheduling Algorithm

The asynchronous die algorithm and Trio algorithm operate togetherwithin each STAR according to the program scheduling algorithm, toschedule program and copy transactions which are to be executed by theNAND Driver.

The Trio algorithm provides information to the asynchronous diealgorithm to define when host data program transactions can beinterleaved with the maintenance copy transactions being created byTrio. Host data program transactions are enabled when the ratio betweenthe number of maintenance metapage copy transactions issued by the Trioalgorithm and the number of host data program transactions issued withinthe current program cycle exceeds the required interleave ratiospredetermined for the program cycle.

An outstanding read transaction for a die has absolute priority in theNAND Driver over any other transaction type for that die. Otheroutstanding transactions for a die are executed in the NAND Driver withrelative priority of 1) program; 2) erase; and 3) copy.

Host data program transactions will be executed ahead of queuedmaintenance copy transactions already present in the NAND Driver.

Address Translation Algorithm

The address translation scheme used in the three layer STAR is asdescribed above where host logical address space may be mappedcontiguously to a separate storage device logical address spaceregardless of the host LBA associated with incoming data.

We claim:
 1. A mass storage memory system, comprising: an interfaceadapted to receive data from a host system; a plurality of flash memorydie, each of the plurality of flash memory die having a plurality oflayers, and each of the plurality of layers in a particular die having adifferent bit-per-cell capacity; and a controller in communication withthe interface and the plurality of flash memory die, the controllerconfigured to: identify an idle die in the plurality of flash memorydie; determine whether a layer in the plurality of layers in the idledie satisfies a background maintenance criterion; when the layer in theidle die is determined to satisfy the background maintenance criterion,execute a background maintenance operation in the layer independentlyand asynchronously of any other operation in other flash memory diemanaged by the controller; and wherein the background maintenancecriterion is based on a ratio of unused data capacity in free blocks inthe layer to a total unused data capacity of the layer, and wherein afree block is a block containing no valid data.
 2. The mass storagememory system of claim 1, wherein the controller is configured toexecute the background maintenance operation by only relocating validdata between blocks in the layer that satisfies the backgroundmaintenance criterion.
 3. The mass storage system of claim 2, whereinthe controller is configured to identify a particular die in theplurality of flash memory die as the idle die only when no host datawrite transactions are queued for execution in the particular die and nomaintenance operations are active in the particular die.
 4. The massstorage system of claim 1, wherein the controller is configured todetermine that the layer satisfies the background maintenance criteriononly when the ratio is less than a threshold ratio for the layer.
 5. Themass storage memory system of claim 4, wherein the backgroundmaintenance criterion is further based on a total amount of valid datain the layer, and wherein the controller is configured to determine thatthe layer satisfies the background maintenance criterion unless thetotal amount of valid data in the layer is greater than a predeterminedmaximum amount for the layer, or the ratio is greater than the thresholdratio for the layer.
 6. The mass storage memory system of claim 4,wherein a first layer and a second layer of the plurality of layers inthe flash memory die each comprise a different threshold ratio.
 7. Themass storage memory system of claim 3, wherein the layer comprises aplurality of partitions, each of the plurality of partitions in thelayer comprising programmed blocks having only a respectivepredetermined data type; and wherein the controller is furtherconfigured to only relocate valid data in the layer by selecting aparticular partition in the layer and only relocating valid data betweenblocks in the particular partition in the layer.
 8. The mass storagememory system of claim 7, wherein the controller is configured to selectas the particular partition a partition in the layer of the idle diehaving a lowest average valid data count per programmed block.
 9. Amethod of performing background maintenance procedures in flash memory,the method comprising: in a mass storage memory system having aninterface for receiving data from a host system, a plurality of flashmemory die, and a controller in communication with the interface and theplurality of flash memory die, the controller: identifying an idle diein the plurality of flash memory die, where each of the plurality offlash memory die include a plurality of layers, and each of theplurality of layers in a particular die having a different bit-per-cellcapacity; determining whether a layer in the plurality of layers in theidle die satisfies a background maintenance criterion; in response todetermining that the layer satisfies the background maintenancecriterion, executing a background maintenance operation in the layerindependently and asynchronously of any other operation in other flashmemory die managed by the controller; and wherein the backgroundmaintenance criterion is based on a ratio of unused data capacity infree blocks in the layer to a total unused data capacity of the layer,and wherein a free block is a block containing no valid data.
 10. Themethod of claim 9, further comprising executing the backgroundmaintenance operation by only relocating valid data between blocks inthe layer that satisfies the background maintenance criterion.
 11. Themethod of claim 10, wherein identifying the idle die comprisesidentifying a particular die in the plurality of flash memory die as theidle die only when no host data write transactions are queued forexecution in the particular die and no maintenance operations are activein the particular die.
 12. The method of claim 9, further comprisingdetermining that the layer satisfies the background maintenancecriterion only when the ratio is less than a threshold ratio.
 13. Themethod of claim 12, wherein the background maintenance criterion isfurther based on a total amount of valid data in the layer, and whereindetermining whether the layer satisfies the background maintenancecriterion comprises determining that the layer satisfies the backgroundmaintenance criterion unless the total amount of valid data in the layeris greater than a predetermined maximum amount for the layer, or theratio is greater than the threshold ratio for the layer.
 14. The methodof claim 12, wherein a first layer and a second layer of the pluralityof layers in the flash memory die each comprise a different thresholdratio.
 15. The method of claim 11, wherein the layer comprises aplurality of partitions, each of the plurality of partitions in thelayer comprising programmed blocks having only a respectivepredetermined data type; and further comprising only relocating validdata in the layer by selecting a particular partition in the layer andonly relocating valid data between blocks in the particular partition inthe layer.
 16. The method of claim 15, wherein selecting the particularpartition comprises selecting a partition in the layer of the idle diehaving a lowest average valid data count per programmed block.